One pervasive feature of daily life in modern industrialized societies is the use of paper products for a variety of purposes. Paper towels, facial tissues, toilet tissue, and the like are in almost constant use. The large demand for such paper products has created a demand for improved versions of the products and of the methods of their manufacture. Despite great strides in papermaking, research and development efforts continue to be aimed at improving both the products and their processes of manufacture.
Paper products such as paper towels, facial tissues, toilet tissue, and the like are made from one or more webs of tissue paper. If the products are to perform their intended tasks and to find wide acceptance, they, and the tissue paper webs from which they are made, must exhibit certain physical characteristics. Among the more important of these characteristics are strength, softness, and absorbency.
Strength is the ability of a paper web to retain its physical integrity during use.
Softness is the pleasing tactile sensation customers perceive when they crumple the paper in their hands and when they use the paper for its intended purposes.
Absorbency is the characteristic of the paper which allows it to take up and retain fluids, particularly water and aqueous solutions and suspensions. In evaluating the absorbency of paper, not only is the absolute quantity of fluid a given amount of paper will hold significant, but the rate at which the paper will absorb the fluid is also important. In addition, when the paper is formed into a product such as a towel or wipe, the ability of the paper to cause a fluid to be taken up into the paper and thereby leave a dry wiped surface is also important.
Processes for the manufacturing of paper products for use in tissue, toweling and sanitary products generally involve the preparation of an aqueous slurry of paper fibers and then subsequently removing the water from the slurry while contemporaneously rearranging the fibers in the slurry to form a paper web. Various types of machinery can be employed to assist in the dewatering process.
Currently, most manufacturing processes either employ machines which are known as Fourdrinier wire papermaking machines or machines which are known as twin wire paper machines. In Fourdrinier wire papermaking machines, the paper slurry is fed onto the top surface of a traveling endless belt, which serves as the initial papermaking surface of the machine. In twin wire machines, the slurry is deposited between a pair of converging forming wires in which the initial dewatering and rearranging in the papermaking process are carried out.
After the initial forming of the paper web on the Fourdrinier wire or forming wires, both types of machines generally carry the paper web through a drying process or processes on another piece of papermaking clothing in the form of an endless belt which is often different from the Fourdrinier wire or forming wires. This other clothing is sometimes referred to as a drying fabric or belt. While the web is on the belt, the drying or dewatering process can involve vacuum dewatering, drying by blowing heated air through the paper web, a mechanical processing in combination with a papermaking felt and subsequent compaction of at least a portion of the paper web.
Vacuum dewatering of the paper web is usually performed by a vacuum apparatus, which is used for applying a fluid pressure differential to the embryonic web. The forming wire carries the web from the forming section to a pick-up shoe, and then to a vacuum box. The pick-up shoe pulls water into the web from the wire, and then out of the web into the belt. The belt takes the web away from the wet transfer point to the press section. The pick up shoe transfers the web from the wire to the belt by vacuum applied through a pick up shoe vacuum slot.
An example of paper webs which have been widely accepted by the consuming public are those made by the process described in U.S. Pat. No. 3,301,746 issued to Sanford and Sisson on Jan. 31, 1967. Other widely accepted paper products are made by the process described in U.S. Pat. No. 3,994,771 issued to Morgan and Rich on Nov. 30, 1976 and U.S. Pat. No. 4,191,609 issued to Trokhan on Mar. 4, 1980. Despite the high quality of products made by these two processes, however, the search for still improved products has, as noted above, continued.
A commercially significant improvement was made upon the above paper webs by the process described in the commonly assigned U.S. Pat. No. 4,529,480 issued to Trokhan on Jul. 16, 1985, which is incorporated by reference herein. The improvement included utilizing a papermaking belt (which was termed a "deflection member") comprised of a foraminous woven member which was surrounded by a hardened photosensitive resin framework. The resin framework was provided with a plurality of discrete, isolated, channels known as "deflection conduits." The process in which this deflection member was used involved, among other steps, associating an embryonic web of papermaking fibers with the top surface of the deflection member and applying a vacuum or other fluid pressure differential to the web from the backside (machine-contacting side) of the deflection member. The papermaking belt used in this process was termed a "deflection member" because the papermaking fibers would be deflected into and rearranged into the deflection conduits of the hardened resin framework upon the application of the fluid pressure differential. By utilizing the aforementioned improved papermaking process, as noted below, it was finally possible to create paper having certain desired preselected characteristics.
The paper produced using the process disclosed in U.S. Pat. No. 4,529,480 is described in the commonly assigned U.S. Pat. No. 4,637,859, issued in the name of Trokhan, which is incorporated herein by reference. This paper is characterized by having two physically distinct regions distributed across its surfaces. One of the regions is a continuous network region which has a relatively high density and high intrinsic strength. The other region is one which is comprised of a plurality of domes which are completely encircled by the network region. The domes in the latter region have relatively low densities and relatively low intrinsic strengths compared to the network region.
The paper produced by the process described in U.S. Pat. No. 4,529,480 was stronger, softer, and more absorbent than similar paper produced by the preceding processes as a result of several factors. The strength of the paper produced was increased as a result of the relatively high intrinsic strength provided by the continuous network region. The softness of the paper produced was increased as a result of the provision of the plurality of low density domes across the surface of the paper.
Although the aforementioned improved process worked quite well, it has been found that when the deflection member of the above-described process passed over vacuum dewatering equipment (vacuum pick up shoe and vacuum box) used in the papermaking process, certain undesirable events occurred. Of most concern is the large number of partially dewatered mobile fibers in the paper web which pass completely through the deflection member. This leads to the undesirable clogging of the vacuum dewatering machinery with the more mobile paper fibers. Another undesirable occurrence is the tendency of these mobile paper fibers to accumulate on the dewatering machinery until clumps of fibers are created. This accumulation of fibers causes papermaking belts which have smooth backsides to wrinkle and develop folds, particularly longitudinal folds. The folds cause severe problems with the moisture and physical property profiles of the paper and eventual failure of the papermaking belt.
The issues which developed when using the smooth backsided papermaking belts in combination with the vacuum equipment having a smooth surface have been at least partially the result of the extremely sudden application of vacuum pressure to the paper web when it passes over the vacuum dewatering machinery. The smooth backside surface of papermaking belt combined with the smooth surface of the vacuum dewatering machinery temporarily create a seal over the vacuum source. Then, when the open channels (the deflection conduits) of the papermaking belt are encountered, the vacuum pressure is very suddenly applied to the highly mobile fibers situated on top of the resin framework. This sudden application of the vacuum pressure is believed to cause the sudden deflection of the mobile fibers which causes them to pass completely through the papermaking belt. It is also believed that this sudden application of vacuum pressure and migration of fibers account for pin-sized holes in the dome regions of the finished paper (or pinholing), which are usually undesirable.
The commonly assigned U.S. Pat. No. 5,334,289 issued to Trokhan et al. on Aug. 2, 1994, and incorporated by reference herein, discloses an improved papermaking belt and a method of making the same, which mitigate the undesirable phenomena of pinholing and buildup of the mobile papermaking fibers on the vacuum dewatering machinery. The disclosed papermaking belt has a backside comprising a network with passageways that provide surface texture irregularities in the backside network. The passageways allow air to enter between the backside surface of the papermaking belt and a web-facing surface of the vacuum apparatus. It is believed that this entry of air significantly reduces or even eliminates the vacuum seal between the backside surface of the belt and the web-facing surface of the vacuum apparatus and, as a result, provides a gradual, or more incremental deflection of the fibers in the embryonic web.
Still, a search for improved products has continued.
It is an object of the present invention to provide an improved papermaking process in which the migration of the aforementioned mobile paper fibers is substantially reduced.
It is another object of the present invention to provide a papermaking vacuum apparatus which will significantly mitigate or eliminate the undesirable application of a sudden vacuum pressure to the paper web.
It is another object of the present invention to provide a papermaking vacuum apparatus which will substantially reduce the problem of the buildup of paper fibers on the vacuum dewatering machinery.
It is also an object of the present invention to provide a papermaking vacuum apparatus which will help to significantly reduce pin-sized holes in the finished paper web (unless such holes are a desirable characteristic for the particular paper being produced).
These and other objects of the present invention will be more readily apparent when considered in reference to the following description and when taken in conjunction with the accompanying drawings.